<!DOCTYPE html>
<html>
<head>
  <meta name="viewport" content="width=device-width">
  <style>
    fencedframe {
      width: 314px;
      height: 271px;
    }
  </style>
</head>
<body>
  <fencedframe></fencedframe>
  <script>
    const url = new URL("viewport_child.html", location.href);
    document.querySelector("fencedframe").config = new FencedFrameConfig(url);
  </script>
</body>
</html>
